The invention relates to a surface pattern made of diffractive structures embedded in a laminate or foil which may be attached to a substrate.
Surface patterns of this type are suitable as a security and authenticity feature on documents, for example bank notes, securities, identification documents, passports, credit cards and other identity cards, smart cards in every form, etc.
Patent applications EP-A 105,099, EP-A 330,738, EP-A 375,833 and EP-A 537,439 disclose optically diffractive security features, embossed in plastic and having N patterns and extremely fine filigree structures, for example guilloches, the visual content of which changes in a predetermined fashion when rotated about an axis. These security features lack machine-readable information.
Security features of this type are embedded in a plastic laminate (EP-A 401,466) and are intended for attaching to a substrate.
Further, volume holograms are known (H. J. Caulfield: xe2x80x9cHandbook of Optical Holographyxe2x80x9d, pages 202-204, 228-229, Academic Press, Inc., 1979, ISBN 0-12-165350-1). Starting with a light sensitive film or foil the hologram is generated by illumination by reference waves and object waves originating from a laser. The interference between the reference waves and object waves generates local variations of the index of refraction within the foil locally in form of closely staggered fringes within the thick and light sensitive layer. The film is practically transparent after the development of the film and at the same time the light entering the foil will be diffracted with a high efficiency by the fringes.
Swiss patent specification CH-PS 653,160 discloses a security document having machine-readable optical authenticity markings. The document has only the authenticity marking which consists of diffraction structures that are composed of joined-together sub-areas with predetermined diffraction structures. It is proposed to partition a coarse relief for a diffraction structure over a plurality of surface parts. However, the information content can easily become manipulated if the authenticity marking is cut up and joined back together again. One important feature is the simultaneous static detection of the information from the entire authenticity marking when the entire surface of the authenticity feature is illuminated with collimated light.
The security feature can have first-type diffraction elements, which according to EP-A 360,969 differ from the other diffraction elements which surround them only by their 180xc2x0 greater azimuth value (asymmetry). With such mirror-symmetric pairs comprising the diffraction elements, it is possible to integrate an item of information into the authenticity feature. A surface pattern of this type has the same disadvantage as Swiss patent specification CH-PS 653,160.
The object of the invention is to provide a surface pattern in the form of an optically diffractive security feature, which has easily readable information for identifying the security feature concealed within the surface pattern and which is difficult or impossible to manipulate.
Said object is achieved according to the invention by adding an arrangement of surface parts to N visible surface patterns of the security feature. The arrangement is independent of the N surface patterns and has a predetermined structure made of its surface parts which is not visible to the unaided human eye. The area of each of the surface parts is covered by a single microscopically fine diffraction structure or by a structure having light absorbing or light scattering properties, or is a plane mirror. The information conveyed by the arrangement of the surface parts determines the different structures used within the arrangement.
Said surface pattern attached to document may be used as a security element which identifies the document in a reading machine.
Illustrative embodiments of the invention will be explained in more detail below with reference to the figures.